The present invention relates to a novel catalyst composition comprising a crystalline metal oxide component, a method of making the composition, and a hydrocarbon conversion process using the composition. More specifically, the invention relates to a phosphate- or vanadate-based crystalline catalyst with nickel incorporated into the crystalline metal oxide framework, deposited thereon, or both. The catalyst is used for the production of synthesis gas from light hydrocarbons (e.g. methane).
The combustion stoichiometry of methane gas at 1000xc2x0 F. is highly exothermic and produces CO2 and H2O according to the following reaction:
CH4+2O2xe2x86x92CO2+2H2O (xe2x88x92190.3 kcal/g mol CH4)
The formed gases are not useful for the production of valuable chemical compounds, and the stability of these products complicates their conversion to more desirable products. Also, further processing is problematic due to the high temperatures generated in the combustion reaction, presenting problems with respect to downstream reactors and catalysts.
In contrast, useful gases, known as synthesis gases or syngases, are produced from the coversion of methane and light hydrocarbons to a mixture containing CO and H2. Conventional syngas-generating processes include the gas phase partial oxidation process (U.S. Pat. No. 5,292,246), the autothermal reforming process (U.S. Pat. No. 5,492,649), and various other processes involving CO2 or steam reforming. These reactions are all significantly less highly exothermic than combustion, and the choice of a particular route depends primarily on the desired product composition, as determined by its end use. Syngas is typically used to produce methanol, ammonia, or heavier hydrocarbon fuels through Fisher-Tropsch technology.
The partial oxidation of hydrocarbons can generally proceed according to several pathways, depending upon the relative proportions of the hydrocarbons and oxygen in the reaction mixture as well as the conditions used. In the case of methane, for example, the following reactions are possible:
2CH4+2O2xe2x86x922CO+2H2+2H2O (xe2x88x9264 kcal/g mol CH4)
2CH4+1.5O2xe2x86x922CO+3H2+H2O (xe2x88x9234.9 kcal/g mol CH4)
or
2CH4+O2xe2x86x922CO+4H2 (xe2x88x925.7 kcal/g mol CH4)
The last reaction is the most desirable in terms of both the quality of the syngas produced and the minimization of liberated heat to protect the apparatus and catalyst bed from thermal damage. A further benefit of this pathway is a reduced formation of steam and corresponding increased yield of hydrogen and carbon monoxide. When combined with the reforming reaction, this reaction product provides a high quality syngas. Therefore, partial oxidation is usually optimal when the carbon to oxygen molar ratio (C:O ratio) in the feedstock gas mixture is maximized. Unfortunately, the improvements in product quality at high C:O ratios are largely offset by a greater tendency of coke formation reactions limiting the partial oxidation catalyst life. As a result, operation is normally constrained to C:O ratios that are lower than ideal for product quality purposes, but that provide reasonable catalyst stability. By lower product quality it is meant that downstream operational costs, such as those associated with recycling unconverted syngas, are increased.
In the autothermal reforming process, the methane and oxygen-containing feeds are mixed and reacted in a diffusion flame. The oxidized effluent is then typically passed into a steam reforming zone where the effluent is contacted with a conventional steam reforming catalyst. The catalyst may be present as a simple fixed bed or impregnated into a monolith carrier or ceramic foam. The high temperature in the catalytic reforming zone places great demands on the reforming catalyst in terms of its ability to substantially retain its catalytic activity and stability over many years of use. As in the partial oxidation process, catalyst coking problems associated with autothermal reforming normally require operation at sub-optimal conditions in terms of the feed stock composition and product quality. Specifically, the level of steam injection required to suppress coking is beyond that dictated solely by concerns about optimizing product quality and minimizing utility costs. In terms of the rate of catalyst coke formation, the molar C:H2O ratio for steam reforming is analogous to the C:O ratio used in partial oxidation. Higher values generally mean a greater coking tendency.
According to the autothermal steam reforming process of U.S. Pat. No. 5,492,649, the production of high amounts of carbon or soot in the diffusion flame oxidation step is avoided by mixing the methane gas with the oxidizer gas while swirling the latter at the injection nozzle to provide a large number of mixing points in the diffusion flame. However, this process still causes partial oxidation in the diffusion flame, resulting in over-oxidation and an excessively high temperature effluent. The heat generated can damage the steam reforming catalyst as well as the face of the injector. Published Canadian Patent Application No. 2,153,304 teaches that the formation of soot is avoided or reduced by actually reducing the steam to carbon molar feed ratio, combined with increasing the steam reforming temperature to between 1100-1300xc2x0 C., and/or introducing the gaseous hydrocarbon feed in increments.
The prior art dealing with partial oxidation and reforming is concerned with overcoming undesired side reactions that lead to the formation of catalyst coke. However, the methods employed have largely been confined to process adjustments, such as those discussed in Canadian Patent Application 2,153,304. As mentioned previously, however, the manipulation of reaction compositions and conditions often restrains partial oxidation and reforming operations to the extent that product quality suffers and/or larger equipment and utilities are imposed.
In terms of adjusting catalyst properties to limit coke formation under a wide range of conditions, it is well known that the use of noble metals (e.g. Pt) provides a solution. However, noble metal-containing catalysts are generally cost prohibitive for industrial applications. Instead, catalysts used in the various types of steam reforming processes normally contain a metal component selected from uranium, Group VII metals, and Group VIII metals. These metals may be combined or used with other metals such as lanthanum and cerium. Generally, the metals are supported on thermally stable inorganic refractory oxides. Preferred catalyst metals are the Group VIII metals, particularly nickel. In the case of nickel, essentially any nickel-containing material has been found useful, e.g. nickel supported on alpha-alumina, nickel aluminate materials, nickel oxide. Preferably, supported nickel-containing materials are used.
Support materials include alpha-alumina, aluminosilicates, cement, and magnesia. Alumina materials, particularly fused tabular alumina, are particularly useful as catalyst support. Preferred catalyst supports may be Group II metal oxides, rare earth oxides, alpha-alumina, modified alpha-aluminas, alpha-alumina-containing oxides, hexa-aluminates, calcium aluminate, or magnesium-alumina spinel. In some cases, catalysts are stabilized by addition of a binder, for example, calcium aluminum oxide. It is preferred to maintain a very low level of silicon dioxide in the catalyst, e.g. less than 0.3 wt-% to avoid volatilization and fouling of downstream equipment. The shape of the catalyst carrier particles may vary considerably. Raschig rings 16 mm in diameter and height, and having a single 6-8 mm hole in the middle, are well known in the art. Other physical forms, such as saddles, stars, beads, and spoked wheels are commercially available.
Catalysts comprising calcium nickel phosphates used in dehydrogenation applications are disclosed in U.S. Pat. No. 3,595,808, where a crystalline hydroxyapatite phase has been known to result from the preparation of such catalysts. This phase is described as a hydroxyapatite in which 1 of every 6 to 12 calcium atoms is replaced by nickel. Apatite structures, having the general chemical formula A10(BO4)6X2 where A=Ca, Sr, Ba, Pb, Cd and other rare earth elements, BO4xe2x95x90PO43xe2x88x92, VO43xe2x88x92, SiO44xe2x88x92, AsO43xe2x88x92, CO32xe2x88x92, and X is OHxe2x88x92, Clxe2x88x92, Fxe2x88x92, CO32xe2x88x92, are more thoroughly discussed and described with reference to their synthesis and physical properties by Brown and Constantz, Hydroxyapatite and Related Materials, CRC Press, Inc. (1994).
The preparation and structural characterization of calcium-phosphate and vanadate solid solutions are described by Boechat, Eon, Rossi, Perez, and Gil, Phys. Chem. Chem. Phys., 2000, 2, 4225-4230, with reference to the incorporation of Sr into the composition to yield Ca10xe2x88x92xSrx(PO4)6xe2x88x92s(VO4)s(OH)2 apatite structures. Finally, the conversion of methane to carbon monoxide in the presence of stoichiometric strontium hydroxyapatite is reported by Sugiyama, Minami, Higaki, Hayahi, and Moffat, Ind. Eng. Chem. Res. 1997, 36, 328-334.
In contrast, applicants have unexpectedly found that the problem of coke formation associated with conventional oxidation and reforming catalysts can be significantly mitigated through the use of a novel catalyst comprising a crystalline metal oxide component having a basic metal (e.g. Sr) and a structural component (e.g. PO4 or VO4), and optionally an alkali metal within the crystalline framework. Active nickel is also included in the framework, dispersed on the crystalline metal oxide component, or both. As a result of the reduced coke formation in hydrocarbon oxidation and reforming operations using the catalyst of the present invention, many of the problems noted above relating to operational constraints and the associated cost burdens are overcome.
In one embodiment, the present invention is a catalyst composition for the production of synthesis gas from light hydrocarbons, the catalyst composition comprising a crystalline metal oxide component having a chemical composition on an anhydrous basis expressed by an empirical formula of:
Av(Bt+)wNixD(Guxe2x88x92)yOz
where A is an alkali metal selected from the group consisting of Li+, Na+, K+, Rb+, Cs+, and mixtures thereof, xe2x80x9cvxe2x80x9d is the mole ratio of A to D and varies from 0 to about 2, B is a basic metal, xe2x80x9cwxe2x80x9d is the mole ratio of B to D and varies from about 1 to about 3, xe2x80x9ctxe2x80x9d is the weighted average valence of B and varies from 2 to about 3, xe2x80x9cxxe2x80x9d is the mole ratio of Ni to D and varies from 0 to about 0.5, D is a framework component selected from the group consisting of P+5, V+5, and mixtures thereof, and G is an anionic species selected from the group consisting of OHxe2x88x92, Clxe2x88x92, Fxe2x88x92, CO32xe2x88x92, and mixtures thereof, xe2x80x9cuxe2x80x9d is the average valence of G and varies from 1 to about 2, xe2x80x9cyxe2x80x9d is the mole ratio of G to D and varies from 0 to about 2, and xe2x80x9czxe2x80x9d is the mole ratio of O to D and has a value determined by the equation:
z=xc2xd(v+txc2x7w+2xc2x7x+5xe2x88x92uxc2x7y),
and when B is Ca, xe2x80x9cvxe2x80x9d is not 0, and when xe2x80x9cxxe2x80x9d is 0, the catalyst composition further comprises a nickel component dispersed on the crystalline metal oxide component.
In a preferred embodiment, the present invention is a catalyst as described above where the crystalline metal component has the hydroxyapatite crystal structure.
In another embodiment, the present invention is a process for preparing a catalyst composition comprising a crystalline metal oxide component having a chemical composition on an anhydrous basis expressed by an empirical formula of:
Av(Bt+)wNixD(Guxe2x88x92)yOz
where A is an alkali metal selected from the group consisting of Li+, Na+, K+, Rb+, Cs+, and mixtures thereof, xe2x80x9cvxe2x80x9d is the mole ratio of A to D and varies from 0 to about 2, B is a basic metal, xe2x80x9cwxe2x80x9d is the mole ratio of B to D and varies from about 1 to about 3, xe2x80x9ctxe2x80x9d is the weighted average valence of B and varies from 2 to about 3, xe2x80x9cxxe2x80x9d is the mole ratio of Ni to D and varies from 0 to about 0.5, D is a framework component selected from the group consisting of P+5, V+5, and mixtures thereof, and G is an anionic species selected from the group consisting of OHxe2x88x92, Clxe2x88x92, Fxe2x88x92, CO32xe2x88x92, and mixtures thereof, xe2x80x9cuxe2x80x9d is the average valence of G and varies from 1 to about 2, xe2x80x9cyxe2x80x9d is the mole ratio of G to D and varies from 0 to about 2, and xe2x80x9czxe2x80x9d is the mole ratio of O to D and has a value determined by the equation:
z=xc2xd(v+txc2x7w+2xc2x7x+5xe2x88x92uxc2x7y),
and when B is Ca, xe2x80x9cvxe2x80x9d is not 0, and when xe2x80x9cxxe2x80x9d is 0, the catalyst composition further comprises a nickel component dispersed on the crystalline metal oxide component, the process comprising:
a) reacting a mixture containing reactive sources of B basic metal, optionally Ni, D framework component, and optionally A alkali metal, at a pH from about 8 to about 14 and a temperature and time sufficient to form the crystalline metal oxide component, the mixture having a composition expressed by:
hA2O:jBOt/2:kNiO: D2O5:lN:mH2O
xe2x80x83where N is a mineralizer, xe2x80x9chxe2x80x9d varies from 0 to about 10, xe2x80x9cjxe2x80x9d varies from about 0.10 to about 6.0, xe2x80x9ckxe2x80x9d varies from 0 to about 1.0, xe2x80x9clxe2x80x9dvaries from 0 to about 20, and xe2x80x9cmxe2x80x9d varies from about 40 to about 500,
b) contacting, when xe2x80x9ckxe2x80x9d is 0, the crystalline metal oxide component with an aqueous solution of a nickel salt selected from the group consisting of nickel nitrate, nickel chloride, nickel bromide, nickel acetate, and mixtures thereof, and
c) calcining the crystalline metal oxide component of step (a) or (b) at a temperature from about 600xc2x0 C. to about 1000xc2x0 C. for a period from about 1 to about 10 hours to yield the catalyst.
In yet another embodiment, the present invention is a process for producing synthesis gas comprising reacting a light hydrocarbon and an oxidant at reaction conditions in the presence of a catalyst composition comprising a crystalline metal oxide component having a chemical composition on an anhydrous basis expressed by an empirical formula of:
AvBt+wNixD(Guxe2x88x92)yOz
where A is an alkali metal selected from the group consisting of Li+, Na+, K+, Rb+, Cs+, and mixtures thereof, xe2x80x9cvxe2x80x9d is the mole ratio of A to D and varies from 0 to about 2, B is a basic metal, xe2x80x9cwxe2x80x9d is the mole ratio of B to D and varies from about 1 to about 3, xe2x80x9ctxe2x80x9d is the weighted average valence of B and varies from 2 to about 3, xe2x80x9cxxe2x80x9d is the mole ratio of Ni to D and varies from 0 to about 0.5, D is a framework component selected from the group consisting of P+5, V+5, and mixtures thereof, and G is an anionic species selected from the group consisting of OHxe2x88x92, Clxe2x88x92, Fxe2x88x92, CO32xe2x88x92, and mixtures thereof, xe2x80x9cuxe2x80x9d is the average valence of G and varies from 1 to about 2, xe2x80x9cyxe2x80x9d is the mole ratio of G to D and varies from 0 to about 2, and xe2x80x9czxe2x80x9d is the mole ratio of O to D and has a value determined by the equation:
z=xc2xd(v+txc2x7w+2xc2x7x+5xe2x88x92uxc2x7y),
and when B is Ca, xe2x80x9cvxe2x80x9d is not 0, and when xe2x80x9cxxe2x80x9d is 0, the catalyst composition further comprises a nickel component dispersed on the crystalline metal oxide component.
These and other embodiments and objects will become clearer after the detailed description of the invention.